1. Field of the Invention
The present invention relates to a device for detecting sunbeam incident angle, and particularly to a sunbeam incident angle detecting device for use in circuitry for monitoring the power generated by solar cell paddles mounted on an artificial satellite.
2. Description of the Prior Art
In an artificial satellite, electric power is supplied by a solar battery consisting of solar cells built in the artificial satellite. For a small artificial satellite which consumes a relatively small amount of electric power, sufficient power can be supplied by providing solar cells over the whole outer surface of the satellite body. In small artificial satellites with such a solar cell arrangement, the electric power generated by the solar battery falls within an allowable range irrespective of the attitude of the artificial satellite.
On the other hand, a large artificial satellite with sophisticated functions consuming much more electric power also requires one or more solar cell paddles. One side of each solar paddle is provided with many solar cells. An artificial satellite with this type of solar cell arrangement orbits with its body being fixedly directed toward the earth. The paddles are designed to rotate according to the sunbeam incident angle with respect to the paddle face so that the solar cells on the paddles always catch the incident sunbeams.
The electric power P.sub.G generated by a solar cell paddle is given as follows: EQU P.sub.G .perspectiveto.K.times.S.times..eta..times.P.sub.SI .times.sin .theta.(W) (1)
where K is a constant determined by such factors as temperature, calendar day and season of the year, S is the effective area (m.sup.2) of the solar cell paddle, .eta. is the conversion efficiency of the solar cell, P.sub.SI is the illumination intensity (W/m.sup.2) of the sunbeam, and .theta. is the angle (degrees) between the incident sunbeam and the solar cell surface.
Estimation and evaluation of the electric power generated by the solar cell paddle play decisive roles for the reliable operation of the artificial satellite system. As seen from Equation (1), the incident angle .theta. of a sunbeam with respect to the solar cell surface is an influential factor in determining the power P.sub.G. In practice, the incident angle .theta. is most difficult to estimate, and conventionally it has been analytically estimated by using a large computer. Calculations of estimation programs had to be executed based on information including the satellite orbit factors, the number of days elapsed from the satellite launch day, the calendar day, the satellite attitude, temperature, deterioration coefficient by radiation, etc. However, such a conventional method has involved the following difficulties. That is, the calculation of the incident angle .theta. requires enormous time, and thus in the case of an artificial satellite with a rather short visible time, the amount of the generated power P.sub.G could not be estimated precisely. Further, each time the attitude of the satellite is changed, the sunbeam incident angle .theta. must be re-calculated with the aid of the computer. Accordingly, the conventional method of finding the incident angle .theta. using estimation programs has been unable to cope with such an emergent situation as a sudden change in the paddle attitude.